Just hit the "Lock Transparent Pixels" icon located in your layer panel.
You can lock out a sketch, but it won't work like I think you want it to. The additional painting of a locked out sketch will only appear in the lines of the sketch. On a new layer, you need to block in the sketch entirely and then lock out that layer. Now, any additional painting on that layer will only appear within the original "blocked in" area.
This VIDEO explains that. This guy talks a lot so skip to 2:00